This invention concerns a procedure for molding a container made of a thermoplastic material, whereby a moldable object that has been temperature-treated is conveyed to a blow station that has a blow mold provided for the purpose of contouring the container, and into which mold the temperature-treated moldable object is inserted, and whereby through a feeding in of high-pressure blow air the moldable object is expanded, and also whereby at least one other pneumatic working step is executed with the use of low-pressure air that is taken from a low-pressure air supply, which has a lower pressure level than does the high-pressure blow air.
Moreover the invention also concerns a device for the molding of a container made of a thermoplastic material, where this device has at least one blow station provided for the accommodation of a temperature-treated moldable object, with the station being equipped with a mold whose inside contour is matched to that of the container to be molded, and where this device is equipped with a high-pressure blow-air supply for expanding the moldable object, and in connection with which at least one structural element in the region of the blow station is connected with a low-pressure air supply for its actuation, with this low-pressure supply making low-pressure air available whose pressure level is lower than high-pressure blow air stored up in the region of the high-pressure blow-air supply.
One way that the molding of such a container can take place is, for example, in a situation where first a moldable object made of polyethylene-terephthalate (PET) is made in the injection-molding process, the moldable object is then heated up after an intermediate storage, and subsequently it is conveyed to the blow station. But it is also known how to make containers according to the spray-blowing method, in which without the interposition of a heating process the moldable object is conveyed to the blow station immediately after its production and upon reaching an adequate stability. Finally, it is also known how to make moldable objects out of tubular sections that are sealed in the region of one of their ends and are provided with a suitable nosepiece in the region of their other end.
What is common to all of these procedures is that the moldable object is smaller in outline than the container to be made. Therefore within the blowing station the moldable object is subjected to the action of compressed air, in order to reshape it into the container that is to be made. In this inflation process, in addition to a reduction in wall thickness due to surface expansion an orienting of the material also takes place. The result of this is that the thin walls of the container have a very high shape stability, which makes the container suitable for a multitude of applications.
Various methods are known for carrying out the blowing process. For one thing, it is possible to employ a uniform blow pressure, which is introduced into the moldable object that is to be inflated, and after an adequate shaping this pressure is vented from the finished container against an ambient pressure. It is likewise already known how to first perform a pre-widening of the moldable object by means of a fairly low pressure, in such a way as to already bring this relatively far in the direction of the shape of the container to be made, and only then to undertake the final impressing of the finer contour of the container by means of a higher pressure. In this method as well, after the container is finished the blow air is vented against an ambient pressure.
In the case of heavy-duty blowing machines, whose normal production output may lie in the range of 2,000 bottles per hour to 20,000 bottles per hour and which customarily produce containers with volumes in the range of 0.5 to 3 liters, considerable quantities of compressed air are therefore needed. So powerful compressors are required to provide this compressed air, which for one thing cost a great deal to purchase because of the large size needed for such a power output, and for another thing consume a considerable amount of energy because of the amount of air compression involved.
Another disadvantage of high compressed-air consumption is the fact that in order to have a high production rate it is necessary to allow large air-flow velocities in venting the finished container. But large flow velocities in venting make it necessary to have powerful sound absorbers, which in turn contribute to increasing the production costs. Moreover, despite the use of such sound absorbers a relatively high noise level is generated.
Therefore the problem of the present invention is to improve upon a procedure of the sort mentioned at the beginning in such a way that the consumption of high-pressure blow air is reduced.
This problem is solved in accordance with the invention in such a way that after the moldable object has expanded, high-pressure blow air streaming out of the molded container that has been secured in the blow mold is fed to a low-pressure air supply during a transition phase, and that in the region of the low-pressure air supply a pressure monitoring is performed in order to define a maximum amount of low air pressure to be provided, and that after this transition phase a venting of the container being held in the blow mold is carried out vis-a-vis an ambient pressure.
Another problem of the present invention is to design a device of the sort mentioned at the beginning, such that by means of a small investment in apparatus, a reduction in the consumption of high-pressure blow air can be achieved.
This problem is solved in accordance with the invention in such a way that within the region of a fastening device that hooks up the blow station to the blow-air supply, a switch-over mechanism is positioned that conducts high-pressure blow air into the region of the blow station during a primary blowing phase, and after an end to the primary blowing phase this mechanism develops, during a transition phase, a connection between the blow station and the low-pressure air supply for the purpose of blow-air transfer, and that the blow station is connected to a disengaging element that after the blow process has ended vents the inside space of the container against an ambient pressure, and that the low-pressure air supply has a pressure limiter for monitoring the presence of a maximum pressure for the low-pressure air.
Because of the transfer of high-pressure blow air into the low-pressure region, in a standard operating configuration a separate provision of low-pressure air is not necessary. This relatively large degree of economizing can be implemented just with a small difference in pressures. In the case of a realistic pressure ratio of 2:1, the savings is about 20%. The final venting against the ambient pressure avoids a danger stemming from an inadequate pressure reduction. Because of the pressure limiter, a pre-assignable pressure level is adhered to.
In the process of venting against the ambient pressure, only a relatively small pressure difference still needs to be removed, resulting in a considerable reduction in noise emissions. Before venting against the ambient pressure, an additional pressure transfer can be made also into an operational-air system that has a pressure level below that of the low-pressure air supply for an initial blowing stage.
In order to provide a definite pressure level in the region of the low-pressure air supply and to ensure an independence from various different pressure conditions upon start-up or in the case of malfunctions, it is proposed that the low-pressure air supply be equipped with an auxiliary source of low-pressure air that is separate and independent of a compressed-air transfer from the region of the blow station.
For the execution of the molding of the container, it is proposed that the moldable object be stretched in the direction of a longitudinal axis during its shaping into the container. In this way, good orientation conditions can be produced in the finished container.
In order to help to provide for a suitable material distribution within the region of the container, it is proposed that low-pressure air be fed to the container for pre-expansion purposes before high-pressure blow air is fed in.
In order to adequately fix in place the container within the blow mold, it is proposed that after executing the transfer phase, a pressure level be maintained within the container that corresponds to the pressure level within the low-pressure air supply.
An effective pressure interval for the high-pressure blow air is provided by having the container acted on with a pressure in the range of 25-40 bars by way of the high-pressure blow air supply.
In using the low-pressure air supply for supplying blow air to the container, it is useful for the container to be acted on by the low-pressure air supply with a pressure in the range of 10-25 bars.
Another variant of the procedure consists in using the low-pressure air supply to provide air to pneumatic structural elements that carry out the positioning movements. In such an embodiment, as a rule it is useful to provide a pressure in the range of 2-8 bars for the low-pressure air.
The process time can be optimized by having the transition phase carried out after shaping the contour of the container and before shape stability is produced by cooling. In this way it is possible in particular to already lower the pressure within the container before any venting of the container, to such an extent that the venting phase itself can be arranged to be relatively short. Given a pre-set length for the total production-time interval, the process time itself can be lengthened in this way, since there is a shorter time requirement for the venting. The cooling can be done, for example, by an attachment placed on the mold.
A compact construction can be realized by using a switchable valve for the execution of the pneumatic switch-over processes.
For purposes of a simple electronic control, it is helpful to have the valves designed to be electromagnetic valves.
A control that is independent of electronic components can also be made available by providing a plate cam for the execution of the pneumatic switch-over movements, with this being equipped with air slots subdivided into segments having differing pressurizations, and by having a follower plate with contact holes that rotates in relation to the plate cam.
In order to even out the pressure conditions, it is proposed that a compressed-air cell be positioned in the region of the low-pressure air supply.
It is useful to avoid great fluctuations in pressure within the region of the low-pressure air supply due to the blow-air transfer by positioning in the region of the transfer mechanism a pressure reducer, which at least in this region constitutes the pressure limiter.